<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .box1 {
      font-weight:normal;
    }
    .box2 {
      font-weight:bold;
    }
    .box3 {
      font-weight:lighter;
    }
  </style>
</head>

<body>
  <div class="box1">
    我是div
  </div>
  <div class="box2">
    我是div
  </div>
  <div class="box3">
    我是div
  </div>
</body>

</html>